I'm developing a theory about working. I've really only applied it to writing, but you should feel free to transpose it to your needs and see if it fits. I assume it will.
The theory is this:

When what you're doing is easy, you don't feel like you're doing anything. When what you're doing is hard, you don't feel like doing anything.

That may sound overly simplistic, but it brings about some interesting thoughts on how we define "work." Writing is my work. I won't even put that in quotes anymore, because it's not up for questioning. I write.

The problem comes when I try to decide if I've done enough. On the days where I can get ten pages out without a break, it doesn't feel like I've done enough because it was easy. The other days, the ones where I rewrote a half of a page six times, don't feel like enough either because I have little to show for it.

It's like we're wired to always think we're failing.

Maybe this is a survival of the fittest type urge to always be better. That's possible. I just know that my emotional core can't resolve these feelings of inadequacy into a "you'll do better next time" type of motivation. Instead it becomes "you'll never do good enough."

That's absolutely the last thing an artist needs to hear.

What's the way to shut this voice up? Well, you can use your friends to this end. Make your source of faith in yourself and your work external. See how long it holds up, especially when someone questions or criticizes something you've done.

Instead, somehow, it's got to come from inside of you. You've got to believe that you have, or could have, what it takes or the overwhelming weight of "not enough" will crush you. And it will quiet you.

How you get to this place is your part of the equation. Heal old wounds. Relieve yourself of unfortunate friendships. Cancel your Facebook account for a while. I don't know what it'll take for you, but I know that if you don't surround yourself with the good influences, you'll probably be overwhelmed by the bad.